The slogan for Flake is ''Only the crumbliest flakiest thing in the world'' because it is aimed at children (aged 3-9). Children (in general) like more crumbly and messy things than adults do. I think this is memorable because this one is for kids and kids will keep wanting to have a Flake.

The slogan for Kitkat is ''Have a break. Have a Kitkat'' because it is aimed at adults. Adults need a break sometimes and this slogan makes adults want a Kitkat. I think this is memorable because it is for adults and it makes adults tempted to have a Kitkat.